Superantigens specific for the antigen receptors of B lymphocytes that have properties akin to known superantigens for T-cells have only recently been identified. The purpose of the current program is to take advantage of unexpected new opportunities to genetically engineer B-cell superantigens with enhanced and novel immunologic properties. The impetus came from recent insights into the in vivo mechanisms of B-cell Sag- induced selective supra-clonal B-cell tolerance by forms of staphylococcal protein A (SpA). Our crystallography based identification of the contact residues responsible for SpA-Fab binding has also provided the critical data for the development of efficient phage display based strategies to create novel B-cell superantigens. To extend investigations of this model of B-cell superantigen, novel superantigens will be created based on the SpA scaffold. The aims of these studies are: 1) To exploit phage display and recombinant technologies for creation of new variants of SpA with enhanced VHIII Fab-specific binding properties. 2) To create novel SpA based domains with binding specificities for other VH supra-clonal sets. 3) To evaluate the properties of these new superantigens in proven in vitro and in vivo assays to determine their effects on B-cell repertoire and circulating Ig, and to consider their impact on immune responsiveness to defined antigens. It is anticipated that these novel proteins will be useful experimental reagents for investigations of B-cell repertoire formation and may also have enhanced activities applicable to the treatment of diseases of B-cell autoimmunity and neoplasia.